“Pretty Little Liars" is a hit on TV and social media, garnering 1.6 million Twitter comments for the season finale.

Thirteen-year-old Gracie Marx is obsessed with the ABC Family show “Pretty Little Liars.”

The Beverly Hills, California, middle-schooler posts images related to the series on Facebook Inc.’s Instagram site, texts with friends while watching and has a whiteboard at home counting the days to the Oct. 23 Halloween special. When cast member Ashley Benson posted on Twitter.com that she was at a nearby cupcake shop, Marx convinced her mother to drive her there and found a small army of girls already snapping photos.

“It’s just a good mystery,” Marx said, when asked why she likes the program.

“Liars” and its online marketing represent an effort by the Walt Disney Co. cable network to keep young audiences watching its broadcasts, including commercials. ABC Family executives accomplish that by creating a kind of virtual watercooler where viewers chat before, during and immediately following each episode.

“Watching this show after the fact is like watching the Super Bowl the next morning,” said Robert Thompson, who teaches television and popular culture at Syracuse University in Syracuse, New York. “For scripted programming that’s really tough to do.”

Programmers are in a constant fight for viewers, battling diversions that take people away from the set and recording technology that lets them skip commercials. In the first quarter, the average person cut their TV time by about 3 percent from a year earlier, according to a Nielsen report.

“Liars,” which began airing in 2010, is the most-watched show in ABC Family’s 11-year history. It averaged 3.7 million viewers, including recorded episodes, in its third season from May to August and was the No. 1 scripted show in its target audience of 18-to-34-year-olds, Disney said, citing Nielsen.

It’s a social-media hit, too, garnering 1.6 million Twitter comments for the Aug. 28 season finale, a record for a TV episode, according to researcher Bluefinn Labs.

Technology is a constant feature in the show, which is based on a popular line of young adult novels. The protagonist, a never-seen character named “A,” communicates via text messages with four high-school friends who dodge murderers and entanglements with a string of attractive co-stars.

Cast members are encouraged to reach out to fans via social media, Lucy Hale, one of the stars, said in an interview. She frequently posts pictures of things she likes, such as new shoes and music videos.

“All of us have been doing this from the beginning,” said the 23-year-old, who has more than 1.7 million Twitter followers. “It was like a wildfire.”

Disney monitors online chatter for opportunities to promote the show. After noticing fans posting pictures of the letter “A” in everything from bathwater bubbles to sand, Disney held a contest for the best “A” picture, generating thousands of photos and 2 million Facebook page views. The 17-year-old winner got a trip to a taping.

“This demographic loves to see themselves in the spotlight,” said Danielle Mullin, ABC Family’s vice president of marketing. “You post your art, we tweet to you, the talent tweets you. That couldn’t happen in the days when you were waiting for Shaun Cassidy to send you a signed photo.”

Disney is now renting out its online army to promote other fare, such as Universal Pictures’ recent film “Pitch Perfect.” “Liars” fans were offered an exclusive web-only clip of the TV show if they clicked “like” on the movie’s Facebook page. More than 100,000 obliged.

“I have three daughters who follow everything about that show,” said Doug Neil, senior vice president of digital marketing at Universal Pictures, part of Philadelphia-based Comcast Corp. “They’ve done a great job in using social media to connect to their base.”